Abstract

Coatings of spherical optical microresonators are widely employed for different applications. Here the effect of the thickness of a homogeneous coating layer on the coupling of light from a tapered fiber to a coated microsphere has been studied. Spherical silica microresonators were coated using a 70SiO2 – 30HfO2 glass doped with 0.3 mol% Er3+ ions. The coupling of a 1480 nm pump laser inside the sphere has been assessed using a tapered optical fiber and observing the 1530-1580 nm Er3+ emission outcoupled to the same tapered fiber. The measurements were done for different coating thicknesses and compared with theoretical calculations to understand the relationship of the detected signal with the whispering gallery mode electric field profiles.

Figures (7)

The effective refractive index of the 70SiO2-30HfO2 coated microsphere as a function of the radius of the silica sphere for the fundamental equatorial (n = 0; l-|m| = 0) TE mode at 1560 nm. The refractive index of the sphere is 1.44 and of the coating 1.6. The coating thicknesses, given in μm, are also reported in the figure. The blue lines correspond to the border cases of no coating (blank sphere) or of very thick coating (bulk sphere with n = 1.6). The horizontal dashed line corresponds to the effective refractive index of the propagation mode of a silica fiber taper with a waist of 3 μm. The vertical dashed line correspond to a silica sphere diameter of 155 μm which is the diameter of the sphere coated in our experiment.

The electric field profiles for a 1 μm coating thickness (left) and for the integral of the absolute electric field inside the sphere core, coating and air versus the coating thicknesses (right) for a sphere 155 μm in diameter for the fundamental (n = 0) and two first internal modes (n = 1,2) closest to 1560 nm. The vertical lines in the left part of the figure correspond to the core-coating and air-coating interfaces.

The average integrated luminescence in the 1535-1585 nm range for the 0.3 mol% Er3+ 70SiO2-30HfO2 coated sphere in respect to the number of dips and to the percentage of the fundamental WGM electric field (e. f.) inside the coating as calculated from the number of dips.

The effective refractive index of the 70SiO2-30HfO2 coated microsphere as a function of the radius of the silica sphere for the fundamental equatorial (n = 0; l-|m| = 0) TE mode at 1560 nm and for the fundamental and the first two internal modes for a 1 μm coating thickness. The refractive index of the sphere is 1.44 and of the coating 1.6. The horizontal dashed line corresponds to the effective refractive index of the propagation mode of a silica fiber taper with a waist of 3 μm. The vertical dashed line correspond to a silica sphere diameter of 155 μm which is the diameter of the sphere coated in our experiment.

The theoretical calculation of the free spectral range for the fundamental (n = 0) and the first two internal modes (n = 1,2) around 1.56 μm for a sphere diameter of 155 μm in respect to the coating thickness and the experimentally measured values of the free spectral range (Fig. 4). The experimental bars in the image are the OSA resolution.